On-chip optical signal routing

ABSTRACT

A microchip may include an optical signal routing system. The optical routing system may include a distribution waveguide coupled to a light source and signaling waveguides interconnecting source and destination locations. A directional coupler may be used to couple and modulate light from the distribution waveguide to a signaling waveguide at a source location. A photodetector may be used to convert light signals from the source location into electrical signals at the destination.

BACKGROUND

[0001] Integrated circuits (ICs) may include signal lines which traversea large portion of the chip. For example, global signal lines may spannearly the entire length of the chip. Electrical repeaters may beincluded in a long signal line to compensate for the lossy nature of theelectrical lines. However, the repeaters may increase the signal delayand power consumption of the chip. These problems may worsen at higherspeeds.

[0002] Electrical lines may be sensitive to electromagnetic interference(EMI), and care must taken to properly shield the lines. Since longsignal lines are typically placed in the upper metallization layers, viablockage may occur when the repeaters are connected to the transmissionline. The EMI interference and via blockage may complicate the design ofthe global signal lines.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003]FIG. 1 is a block diagram of a microchip including an on-chipoptical signal routing system.

[0004]FIG. 2 is a flowchart describing an optical signal routingoperation.

[0005]FIG. 3 is a sectional view of a vertical cavity surface emittinglaser (VCSEL).

[0006]FIG. 4 is a sectional view of an integrated waveguide structure.

[0007]FIG. 5 is a plan view of the integrated waveguide structure.

[0008]FIG. 6 is a schematic diagram of a directional coupler.

DETAILED DESCRIPTION

[0009]FIG. 1 shows a microchip 100 including an on-chip optical signalrouting system. The on-chip optical signal routing system may transfersignals over relatively large distances on the chip using modulatedlight beams. In typical chips, electrical signals may be carried byrelatively long electrical signal lines, e.g., global signal lines. Longelectrical lines typically include electrical repeaters at intervals tocompensate for signal attenuation due to the lossy nature of theelectrical lines. However, various problems may be associated with theinclusion of repeaters, including, for example, increased signal delayand power consumption, electromagnetic interference (EMI), and viablockage between metallization layers in the chip.

[0010] The on-chip optical routing system may include a continuous wave(CW) light source 105 coupled to a distribution waveguide 110. Thedistribution waveguide 110 may act as a “light pipe” which provides areservoir of photons for on-chip signaling. The distributed waveguide isdistributed across the chip. As shown in FIG. 1, the distributionwaveguide 110 may form a winding with an end that converges back intoitself to form a closed ring. This closed loop configuration mayminimize power fluctuations in the ring. Alternatively, the distributedwaveguide 110 may be distributed across the chip by successive splittingand fanning out of the distribution waveguide into branches.

[0011]FIG. 2 is a flowchart describing an optical signal routingoperation 200. Electrically controlled modulated taps 115 may beprovided at origin points (e.g., Point A) to tap light off of thedistribution waveguide 110 (block 205) and modulate the tapped light.The modulated taps may be activated by applying a control voltage. Whenthe voltage is applied, some of the light in the distribution waveguidemay be transferred out of the distribution waveguide and into asignaling waveguide 120. When the voltage is removed, the light is onceagain blocked from being transferred into the signal waveguide. Pulsesof light can be made to travel down the signal waveguide by successivelyapplying the control voltage to the modulated tap, i.e., turning themodulated tap on and off to produce a desired signal pattern (block210). These pulses may then be detected at a destination point (e.g.,Point B) at the end of the signaling waveguide 120 by a photodetector orphototransistor 125 (block 215), which may convert the light signalsback to electrical signals (block 220). The electrical signals may betransferred to electronic circuitry in the microchip relatively near thedestination point (block 225).

[0012] The CW light source 115 may be, for example, an optical fiber,edge-emitting laser, vertical cavity surface emitting laser (VCSEL), orother semiconductor laser. VCSELs may be desirable for their uniform,single mode beam profiles, which may be more easily coupled to opticalfibers. The cavity length of VCSELs may be very short, e.g., one tothree wavelengths of the emitted light. As a result, a photon may have asmall chance of triggering a stimulated emission event in a single passof the cavity at low carrier densities. Consequently, VCSELs may requirehighly reflective mirrors to be efficient. The reflectivity of thefacets in edge-emitting lasers may be about 30%, whereas, for VCSELs,the reflectivity required for low threshold currents may be greater than99%. Achieving such a high reflectivity with metallic mirrors may beimpractical. Instead, many VCSELs use Distributed Bragg Reflectors(DBRs). FIG. 3 shows an exemplary VCSEL structure 300. DBRs 305 in thelaser structure may be formed by laying down alternating layers ofsemiconductor or dielectric materials with different refractive indexes.

[0013] The distribution waveguide 110 and the signaling waveguides 120may be integrated in the chip. A cross section and a top view of anintegrated waveguide are shown in FIGS. 4 and 5, respectively. Thewaveguide may include an optically guiding core 405 of a material withrefractive index n_(w) surrounded by a cladding material with adifferent index of refraction, n_(c). The high contrast of therefractive index between the two materials provides nearly completeinternal reflection in the core, thereby confining a lightwave to thewaveguide 405.

[0014] Silicon oxide (SiO₂) (n_(c)≈1.5) may be used as the claddingmaterial. The waveguide material may be selected from, e.g., siliconnitride (Si₃N₄) (n_(w)≈2), silicon (Si) (n_(w)≈3), and siliconoxynitride (SiON) (n_(w)≈1.55). Silicon oxynitride may offer designflexibility because its refractive index may be varied by changing thecontent of nitrogen.

[0015] The waveguides may be classified as high index contrast (HIC) orlow index contrast (LIC) depending on the difference in the indices ofrefraction between the core and the cladding. In a HIC waveguide, coreand cladding materials are chosen to have very different indices ofrefraction, e.g., n_(w)≈2.0 and n_(c)≈1.5. This, in turn, may cause theelectric field to be strongly confined within the core, substantiallyreducing radiation loss for sharp bends (e.g., less than about 50microns) and allowing smaller structures to be produced. The LICwaveguides may have a smaller contrast between the core and claddingindices of refraction, e.g., n_(w)≈1.6 and n_(c)≈1.5. The distributionwaveguide may be a relatively large LIC waveguide with bends havingrelatively large radius of curvatures (e.g., about 1 mm). The signalingwaveguides 120 may be smaller HIC waveguides, which may includerelatively sharp bends.

[0016] Driver circuits 602 in the microprocessor 105 may drive themodulated taps to control and modulate the CW light from thedistribution waveguide 110 for on-chip signaling. The modulated taps 115may work at high frequency to both tap light from the distributionwaveguide 110 and into signaling waveguides 120 and encode data bymodulating the tap.

[0017] The modulated taps 115 may couple light from the distributionwaveguide 110 into the signaling waveguides 120. “Mode” refers to thesolution of Maxwell's wave equation satisfying the boundary conditionsof a waveguide, thus forming a unique pattern of standing wave in theradial direction on the cross section of the waveguide. A mode ischaracterized by its propagation constant (eigenvalue of the waveequation). Evanescent coupling may occur when the evanescent tails ofeach waveguide overlap to such a degree that there are two possiblesolutions for mode propagation in the two waveguide structure. These maybe referred to as the “Supermodes” or “Eigenmodes.” The two solutionsmay have symmetric and antisymmetric energy distributions and differingpropagation constant values. As the relative phases of the modes change,the energy is shared between the two waveguides and at matching andmismatched phase, the energy is alternately maximized in each waveguide,i.e., the energy beats back and forth between the waveguides, dependenton the waveguide separation and the interaction length.

[0018] As shown in FIG. 6, a modulated tap 115 may include twoside-by-side waveguides (e.g., distribution waveguide 110 and asignaling waveguide 120) separated by a few tenths of micrometers to afew micrometers. Voltage applied by an electrode 605 may cause a changein the evanescent coupling efficiency between waveguide 110 andwaveguide 120. In the off state the light goes through the deviceunaltered, i.e., no light is tapped from the distribution waveguide 110.When a high frequency signal voltage is applied, the intensities at theoutput ports 610 are determined by either modulation of the phasemismatch, Δβ, or the coupling coefficient K. Thus, change of voltage byan amount V_(s) switches an input signal from one output port to theother. The now modulated light is transferred to the signaling waveguideand is sent off-chip. Only a portion of the light in the distributionwaveguide 110 may be needed, e.g., about 5%. Since all of the light isnot being switched to the signaling waveguide 120, a full π phase shiftmay not be required.

[0019] The integrated waveguides may be fabricated on a silicon layer inthe chip. For example, a lower cladding layer may be formed by thermaloxidation of the silicon layer. The core may be deposited by plasmaenhanced chemical vapor deposition (PECVD). A waveguide pattern may bedefined by optical contact lithography and transferred to the core layerby reactive ion etching (RIE). The etched waveguide pattern may beovergrown with PECVD silicon oxide as the upper cladding layer.

[0020] The optical components may be incorporated in optics layer(s),which may be separate from the layers containing the electroniccircuitry components of the microprocessor. For example, the opticallayer(s) may be formed on the top metallization layer of the chip duringbackend processing. In this case, a lower cladding layer for theintegrated waveguides may be formed by growing a silicon oxide layerusing chemical vapor deposition (CVD) or sputtering techniques.

[0021] The use of optical signal lines (waveguides) over relativelylarge distances may have several advantages over electrical signallines. Signal delay and power dissipations may be reduced by eliminatingelectrical repeaters. Die area and via blockage may also be reduced byeliminating the need for repeaters on signal lines. Furthermore, signalwaveguides may intersect without significant crosstalk, therebysimplifying layout design.

[0022] A number of embodiments have been described. Nevertheless, itwill be understood that various modifications may be made withoutdeparting from the spirit and scope of the invention. Accordingly, otherembodiments are within the scope of the following claims.

1. An apparatus comprising: a microchip including a distributionwaveguide adapted to be coupled to a light source, a signaling waveguidehaving an end proximate a portion of the distribution waveguide, and amodulated tap to couple light from the distribution waveguide into thesignaling waveguide and modulate the light into light signals.
 2. Theapparatus of claim 1, further comprising a converter coupled to anotherend of the signaling waveguide and operative to convert the lightsignals into electrical signals.
 3. The apparatus of claim 2, whereinthe microchip further comprises: a plurality of layers includingelectronic circuitry, the circuitry; and an interconnect between theconverter and the electronic circuitry in the microchip.
 4. Theapparatus of claim 2, wherein the converter comprises a photodetector.5. The apparatus of claim 2, wherein the converter comprises aphototransistor.
 6. The apparatus of claim 1, wherein the light sourcecomprises a single mode light source.
 7. The apparatus of claim 1,wherein the light source comprises a semiconductor laser.
 8. Theapparatus of claim 1, wherein the distribution waveguide comprises a lowindex contrast waveguide and the signaling waveguide comprises a highindex contrast waveguide.
 9. The apparatus of claim 1, wherein thewaveguides comprise silicon waveguides.
 10. The apparatus of claim 9,wherein the waveguides comprise a core including silicon and a silicacladding layer.
 11. The apparatus of claim 1, wherein the microchipfurther comprises: a plurality of layers including electronic circuitry,the circuitry including a driver operative to drive the modulated tap,and an interconnect between the driver and the modulated tap.
 12. Theapparatus of claim 1, wherein the modulated tap comprises a directionalcoupler.
 13. A method comprising: coupling light from a distributionwaveguide on a microchip to a signaling waveguide in the microchip;modulating light in the signaling waveguide to produce light signals;and converting the light signals to electrical signals at a destinationon the microchip.
 14. The method of claim 13, wherein said modulatingcomprises modulating the light in response to electrical signals. 15.The method of claim 13, wherein said modulating comprises coupling anddecoupling light from the distribution waveguide to the signalingwaveguide.
 16. The method of claim 13, further comprising transferringthe electrical signals to electronic circuitry in the microchip.
 17. Themethod of claim 13, wherein said coupling comprises coupling light by anevanescent coupling effect.
 18. The method of claim 13, wherein saidcoupling comprises providing a control voltage to a directional couplerat a junction between the distribution waveguide and the signalingwaveguide.
 19. A method comprising: coupling light to a distributionwaveguide in a microchip; modulating at least a portion of the light toproduce light signals at a source location on the microchip;transferring the light signals to a destination location on themicrochip; and converting the light signals into electrical signals. 20.The method of claim 19, further comprising transferring the electricalsignals to electronic circuitry proximate the destination location inthe microchip.
 21. The method of claim 19, wherein said transferringcomprises coupling the modulated light to a signaling waveguideinterconnecting the source location and the destination location.
 22. Asystem comprising: a light source; and a microchip including anintegrated distribution waveguide coupled to the light source, a sourcelocation on the distribution waveguide including a modulated tap tomodulate at least a portion of the light in the distribution waveguideinto light signals, a destination location including a converter toconvert the light signals into electrical signals, and an integratedsignaling waveguide interconnecting the source location and thedestination location.
 23. The system of claim 22, wherein the microchipfurther comprises: a plurality of layers including electronic circuitry,the circuitry; and an interconnect between the converter and theelectronic circuitry in the microchip.
 24. The system of claim 22,wherein the converter comprises a photodetector.
 25. The system of claim22, wherein the converter comprises a phototransistor.
 26. The system ofclaim 22, wherein the light source comprises a single mode light source.27. The system of claim 22, wherein the distribution waveguide comprisesa low index contrast waveguide and the signaling waveguide comprises ahigh index contrast waveguide.
 28. The system of claim 22, wherein thewaveguides comprise silicon waveguides.
 29. The system of claim 22,wherein the microchip further comprises: a plurality of layers includingelectronic circuitry, the circuitry including a driver operative todrive the modulated tap, and an interconnect between the driver and themodulated tap.